1. Field of the invention:
The present invention relates to the production of coherent short wavelength radiation and, more particularly, to the production of coherent short wavelength radiation (e.g., X-rays and/or ultraviolet) utilizing the Compton scattering effect.
2. Description of the Related Art:
A coherent source of X-rays in the energy region of 500 to 1000 eV can be used as a first component of an X-ray microscope capable of very high resolution, to study biological materials in vivo, i.e., in their hydrated form.
This source is the equivalent of the light source in a conventional microscope. Many other applications of an X-ray generator, not necessarily in the field of life sciences, have been reported in the literature.
Quasi-coherent X-ray beams in the water window energy range are produced by storage rings; the synchrotron radiation from these machines is filtered by selecting small angular apertures of the radiation passed through a very small hole, and through a monochromator. As a consequence, the photon flux is reduced enormously (by some seven orders of magnitude or more). In practice, these sources have been used to make proof-of-principle holographic images of simple objects; scanning images and quasi-optical images have been shown as well. The intensity available from storage rings does not allow short exposure times; exposures very from several minutes to several hours (see Table 1 below adopted from M.R. Howells. "Soft X-Ray Imaging for the life Sciences", Advanced Light Source Report, (LSGN-008) LBL-27420)).
TABLE I __________________________________________________________________________ Characteristics of Soft X-Ray Imaging Techniques CONTACT IMAGING SCANNING HOLOGRAPHY __________________________________________________________________________ Resolution Achieved 200 500 500 630 Angstroms Dose (Mrads) 50 10-60 1 200 (At resolution above) Exposure Time Magnet 5 min. 10-100 sec. 1 hr. 1 day Undulator 5 sec. 2 min. 1 hr. Coherence needed? No No Yes Yes ##STR1## 3 300 300 500 Contrast Amplitude Amplitude Amplitude Amplitude and Phase Phase Contrast Not Possible Possible Possible Occurs Naturally Quantitative Poor Potentially doable Differential Poor Microanalysis (Diff. Absorption) Absorption Fluorescence __________________________________________________________________________
Future high brightness storage rings, due to turn on in the next few years, may increase the available fluxes to values whose X-ray microscopy will fulfill the promise of high resolution. But as long as the source of X-ray is a storage ring, the technology of high-resolution imaging will not leave the national laboratory environment.
The importance of observing biological structures in a state, as close as possible to the natural state cannot be emphasized enough. Presently, high-resolution images of biological active, functioning structures are not available; in fact, most of the images are obtained by staining, modifying and denaturing the original materials, so that, in many cases, little resemblance remains to the original object.
Luccio et al., "Coherent Backscattering in the Soft X-Ray Region", BNL 38450 (June 1986) propose a method to generate monochromatic X-rays by scattering a photon beam on a bunch of electrons, whose density in the direction of motion is appropriately modulated (to be in resonance with the Doppler-shifted wavelength cf the incoming laser), a significant increase in the scattered radiation intensity can be obtained. Also, a degree of coherence is present in the scattered beam. The modulation of the bunch is obtained via the interaction of a magnetic field and the laser light itself, giving an energy modulation first This energy modulation is transformed in spatial modulation, as required, by transporting the modulated bunch for some distance.
While the above-described backscattering technique produces X-rays having a narrow bandwidth, the X-rays do not have sufficient spatial coherence, nor do they have sufficient intensity, to make "in vivo" imaging possible at the necessary resolution. Other methods known in the art for producing coherent X-rays, such as using storage rings to produce quasicoherent X-rays (discussed previously), result in extreme attenuation and thus require excessive exposure times for microscopy.